1. Field of Invention:
This invention is in the field of torque control devices, particularly the field of torque control devices designed to monitor and control the amount of torque applied by a tong and cat head system during the make-up and breakout of drill collar and drill pipe, and this invention is designed to provide a system and an apparatus for accurately computing the actual applied torque during make-up and breakout, correcting for variations in the vertical and horizontal angles between the longitudinal axis of the tong and the direction of the cat head pull, shutting down the cat head at the instant the desired make-up torque is achieved, and monitoring the actual applied torque during breakout.
2. Prior Art:
The joints in a drill string must be made up with an applied torque within a rather narrow range of torque values. If a joint is not adequately tightened, the joint may lea, and may ultimately result in a crack or a separation of the drill string causing the lower part of the drill string to fall into the well. A very costly recovery operation would then be required to remove the separated portion of the drill string from the well. However, if the make-up torque is too high, the pipe threads may be damaged or the pipe may be overstressed causing its failure and the loss of a portion of the drill string in the well. Again, this would necessitate an expensive recovery operation and a temporary shut down of the well.
The earliest method for use of a tong and cat head for the make-up and breakout of a drill string consisted of the operator manually observing the angular advancement of the tong and the apparent effort being made by the cat head, to determine, on the basis of experience, when the desired amount of torque was being applied. In the case of make-up, when it appeared that the desired torque was achieved, the operator would manually shut down the cat head. In the case of breakout, the operator would merely observe if the breakout torque was apparently greater than the make-up torque that had been applied, thereby observing whether the drill string had received additional torquing in the well, an undesirable condition.
An early advancement from the completely manual system for controlling torque applied by the tong to the drill string was the use of a strain guage or some other form of force measuring device in the line between the cat head and the tong. This would allow the operator to manually observe when the force applied by the cat head approximately achieved the desired maximum. An additional advancement was accomplished through the use of a load sensor which automatically disengaged the cat head clutch when a preset maximum force was exceeded. The main deficiency with this early automated system wa sthat as the tong arm is rotated, the angle between the axis of the tong arm and the direction of pull of the cat head cable varies. Since it is only that component of the force imposed by the cat head which is perpendicular to the axis of the tong and is in the horizontal plane of the tong which imposes torque upon the drill string, the use of the applied force without an angle correction results in an incorrect torque value. If the preset force is that force which would produce the desired make-up torque when the pre-set force is applied in a direction perpendicular to the axis of the tong arm, then more likely than not, the actual torque applied to the drill string at the time of shutdown will be less than the desired torque, since only infrequently will shutdown occur when the angle between the applied force and the tong arm axis is 90.degree.. If the shutdown force is set higher, then the potential for overtorquing the drill string joint is introduced. Also, an additional lesser problem is that frequently the tong will be attached to the drill string in a different horizontal plane than the horizontal plane that the cat head is situated in, thereby introducing a vertical angle which must be corrected for in order to compute the correct torque.
One of the earliest attempts to deal with the need to correct the torque calculation for the variation in the angle between the direction of the applied force and the axis of the tong is illustrated in U.S. Pat. No. 3,693,727 (Bell). This device contains a lever and sensor arrangement attached to the tong which attempts to provide for the direct sensing of that horizontal component of the force applied to the tong by the cat head which is perpendicular to the axis of the tong arm. Inherent problems with this device are lack of accuracy and lack of vertical angle corrections. Another device which attempts to compensate for the variation of the angle between the horizontal axis of the tong and the direction of the applied force is illustrated in U.S. Pat. No. 4,137,758 (Rodland). This device, like the Bell Patent, includes an attachment for the tong arm which allows the horizontal component of the applied force which is perpendicular to the axis of the tong arm to be measured directly. This device uses a load cell which is fixed between the tong arm and an external pivot point which transmits only the perpendicular component of the applied force to the load cell. The accuracy of this device is questionable and again, like the Bell Patent, it is not designed to correct for vertical angle deviations. Neither the device described by the Bell Patent nor the device described by the Rodland Patent provide for determination of the vertical and horizontal angles or the determination of the force actually applied by the cat head, all of which are important for monitoring operator performance.
The device disclosed in the Gann Patent, U.S. Pat. No. 4,501,335, incorporates a device which attaches to the handle of the tong arm and measures the horizontal angle between the horizontal axis of the tong arm and the direction of the applied force, through the use of a universal assembly with shaft oriented vertically and an optical encoder angle measuring gauge attached to the shaft. The tension force is measured with a typical strain guage. Electrical signals from the horizontal angle encoder and the applied force strain guage are used to compute the applied make-up torque. The accuracy of the angle measurement and the force measurement is questionable for this system and, like the other devices disclosed in the prior art, no correction is made or is possible for the vertical angle deviation.
Another significant deficiency of all prior art devices for torque control is related to the tendency of the cat head to continue to torque up the drill string after the cat head solenoid is disengaged. Although complete shutdown is usually accomplished in a fraction of a second, the time lags and inertia of the system can result in significant overtorqueing of the drill string if shut down is initiated just as the maximum desired torque is achieved.